THE DIGESTIVE SYSTEM

THE DIGESTIVE SYSTEM

Stages in the Digestive Process

1. MOVEMENT : propels food through the digestive system

2. SECRETION : release of digestive juices in response to a specific stimulus

3. DIGESTION : breakdown of food into molecular components small enough to cross the plasma membrane

4. ABSORPTION : passage of the molecules into the body’s interior and their passage throughout the body

5. ELIMINATION : removal of undigested food and wastes

The human digestive system, is a coiled, muscular tube (6-9 meters long when fully extended) stretching from the mouth to the anus. 

The Mouth and Pharynx

Chemical breakdown of starch by production of salivary amylase from the salivary glands into glucose. This mixture of food and saliva is then pushed into the pharynx and esophagus.

The STOMACH

Gastric juice in stomach contains: 

• hydrochloric acid(HCl), 
• pepsinogen, and 
• mucus

Functions of Hydrochloric acid(HCl) :

• It kills microorganisms, 
• It lowers the stomach pH to between 1.5 and 2.5.
• It lowers pH of the stomach so pepsin is activated. 

Pepsinogen is an enzyme that starts protein digestion and controls the hydrolysis of proteins into peptides.

Chyme, the mix of acid and food in the stomach, leaves the stomach and enters the small intestine.

Alcohol and aspirin are absorbed through the stomach lining into the blood.

Epithelial cells secrete mucus that forms a protective barrier between the cells and the stomach acids. 

ULCERS

Peptic ulcers result when these protective mechanisms fail. 

Bleeding ulcers result when tissue damage is so severe that bleeding occurs into the stomach.

Perforated ulcers are life-threatening situations where a hole has formed in the stomach wall.

At least 90% of all peptic ulcers are caused by Helicobacter pylori. 

Other factors, including stress and aspirin, can also produce ulcers.

☞THE SMALL INTESTINE

• The small intestine is the major site for digestion and absorption of nutrients. 
• it is about 22 feet (6.7 meters) long.

Parts of small intestine:

1. Duodenum
2. Jejunum
3. Ileum

Sugars and amino acids go into the bloodstream via capillaries in each villus.

Glycerol and fatty acids go into the lymphatic system.

Starch and glycogen are broken down into maltose by small intestine enzymes.

Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli. 

Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. 

Gluten enteropathy is the inability to absorb gluten, a protein found in wheat.

Fat digestion is usually completed by the time the food reaches the ileum (lower third) of the small intestine. Bile salts are in turn absorbed in the ileum and are recycled by the liver and gall bladder. 

☞LIVER

The liver produces and sends bile to the small intestine via the hepatic duct.

Bile contains cholesterol, phospholipids, bilirubin, and a mix of salts.

In addition to digestive functions, the liver plays several other roles:

(1) detoxification of blood;

(2) synthesis of blood proteins;

(3) destruction of old erythrocytes and conversion of haemoglobin into a component of bile;

(4) production of bile;

(5) storage of glucose as glycogen, and its release when blood sugar levels drop; and

(6) production of urea from amino groups and ammonia.

☞GALL BLADDER

It stores excess bile for release at a later time. 

We can live without our gall bladders, in fact many people have had theirs removed. The drawback, however, is a need to be aware of the amount of fats in the food they eat since the stored bile of the gall bladder is no longer available.

Glycogen is a polysaccharide made of chains of glucose molecules. 

In plants starch stored in the form of glucose, while animals use glycogen for the same purpose. 

Low glucose levels in the blood cause the release of hormones, such as glucagon, that travel to the liver and stimulate the breakdown of glycogen into glucose, which is then released into the blood(raising blood glucose levels). 

When no glucose or glycogen is available, amino acids are converted into glucose in the liver.The process of deamination removes the amino groups from amino acids. Urea is formed and passed through the blood to the kidney for export from the body. Conversely, the hormone insulin promotes the take-up of glucose into liver cells and its formation into glycogen.

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